1. Field of the Invention
This invention relates generally to signal receiving apparatus and, more particularly, to a signal receiving apparatus for receiving signals including error detection codes and that includes a muting mode.
2. Description of the Prior Art
In data transmission there are some cases in which a coded signal containing errors is received by a receiving apparatus, with transmission of the signals having been affected by noise or some other instantaneous disturbance. Though such code errors should preferably be small, the errors cannot be reduced to zero. Thus, in case an error is generated during transmission, the error code thereof is detected and error correction is carried out by a certain method so as to improve transmission quality. On the other hand, if the errors can not be corrected the demodulated signals are not output based on the detected error code.
For example, when RF-modulated pulse code modulation (PCM) digital signals are transmitted from a transmitter side and are demodulated at a receiver side, the error code of the demodulated signals is read and used to mute the demodulated signals if the errors can not be corrected.
In an audio sound system of an aircraft where each group of seats includes a signal receiving apparatus, for example, demodulated signals are muted in accordance with an error code thereof, so that the passengers will not hear audio signals of deteriorated quality.
In such case, the error code judging time, that is, the sampling time, is fixed to the sampling time of the normal operating mode of the audio signal playback apparatus forming part of the aircraft sound system.
Nevertheless, in addition to the normal operating mode in which the passengers enjoy listening to music, there are other operating modes for the aircraft audio sound system, such as, a channel selection mode in which the passengers themselves tune to change the band being listened to, and a Passenger Address (PA) selection mode in which the same announcement is mandatorily made to all the receiving sets of the aircraft sound system.
In this PA selection mode for announcing a message from the cockpit, the voices are outputted through so-called PA speakers throughout the aircraft, while the channel selection status of audio signals for each seat is controlled to switch to a predetermined PA channel, so as to assure delivery of the message to all of the passengers, including those using headphones.
Meanwhile, if the sampling time for the error code is fixed for muting the demodulated signals in accordance with the error code thereof, it is extremely difficult to sufficiently follow detection of the error code in the vicinity of a threshold for determining the error. More specifically, in case there are plural operating modes of the audio signal receiving apparatus, such as the channel selection mode and the PA selection mode, as well as the normal operating mode described above, restricting the sampling of the error codes only to the sampling time of the normal operating mode causes the muting time to be inappropriate for these other operating modes.
As a result of such fixed error code sampling time, in the PA selection mode the mute release is carried out after the voices of the announcement have started, and the message cannot be delivered in full to the listeners. This is the same as the absence of correct digital data. On the other hand, in the normal operating mode, the sampling of the error code in an inappropriately short sampling time causes the timing of the mute release to be too early, thus outputting erroneous digital data.